94 MICROSCOPIC METHODS 



visible on the cover-glass. When the film has been spread, it 

 must next be dried by being waved backwards and forwards at 

 arm's-length above a Bunsen flame. The film must then be fixed 

 on the glass by being passed three or four times slowly through 

 the flame. In doing this a good plan is to hold the cover-glass 



between the right forefinger 

 and thumb ; if the fingers just 

 escape being burned no harm 

 will accrue to the bacteria in 



. FIG. 40. Cornet's forceps for holding ',. p, r ,-L- i 



cover-glasses. ^ n m aking films 01 a thick 



fluid such as pus, it is best to 



spread it out on one cover with the needle. The result will be 

 a film of irregular thickness, but sufficiently thin at many parts 

 for proper examination. Scrapings of organs may be smeared 

 directly on the cover-glasses. 



In the case of blood, a fairly large drop should be allowed to 

 spread itself between two clean cover-glasses, which are then to 

 be slipped apart, and being held between the forefinger and 

 thumb are to be dried by a rapid to-and-fro movement in the 

 air. A film prepared in this way may be too thick at one edge, 

 but at the other is beautifully thin. If it is desired to preserve 

 the red blood corpuscles in such a film it may be fixed by one 

 of the following methods : by being placed (a) in a hot-air 

 chamber at 1 20 C. for half an hour ; (b) in a mixture of equal 

 parts of alcohol and ether for half an hour, then washed and 

 dried ; (c) in formol-alcohol (Gulland) (formalin 1 part, absolute 

 alcohol 9 parts) for five minutes, then washed and dried ; or (d) 

 in a saturated solution of corrosive sublimate for two or three 

 minutes, then washed well in running water and dried. (Fig. 69 

 shows a film prepared by the last method.) In using the 

 Romanowsky stains no previous fixation is necessary (vide infra). 

 In the case of urine, the specimen must be allowed to stand, and 

 films made from any deposit which occurs ; or, what is still 

 better, the urine is centrifugalised, and films made from the 

 deposit which forms. After dried films are thus made from 

 urine it is an advantage to place a drop of distilled water on the 

 film and heat gently to dissolve the deposit of salts ; then w^ash 

 in water and dry. In this way a much clearer picture is 

 obtained when the preparation is stained. 



Within recent years it has become common to make blood 

 films on ordinary microscopic slides instead of upon cover- 

 glasses. Here the slides must be clean. This can be effected by 

 washing thoroughly first with weak alkali and then with water 



